1. Field of the Invention
The present invention relates to a control apparatus for a brake device which can control a displacement of a piston using an electromotive device, such as a motor, and which generates a braking force by moving the piston so as to bring a brake friction member into contact with an object to be braked.
2. Description of the Related Art
Recently, various brake devices that can freely change a pressing force applied to a brake friction member using an electromotive device, such as a motor, have been suggested. A vehicle having such a brake device can perform brake control without being restricted by a brake command from a driver. Therefore, it is possible to improve both the safety in driving the vehicle and comfort. For example, Japanese Unexamined Patent Application Publication No. 2003-175816 discloses an electric brake system capable of synchronizing the times at which brake devices apply brakes to respective wheels. In this system, when it is detected that a brake pad of one of the brake devices has come into contact with a corresponding disc rotor, a braking force obtained at the contact position is maintained while brake pads of other brake devices that are not yet in contact with respective disc rotors are quickly brought into contact with the respective disc rotors.
In general, such a brake device includes an electric actuator and is controlled by a control device that stores a graph representing the relationship between a motor position and a pressing force applied to the pad in advance. The motor is controlled such that the motor position corresponds to a pressing force required by the driver.
On the other hand, Japanese Unexamined Patent Application Publication No. 2002-81475 discloses a method for detecting a motor position corresponding to a contact point at which a pad comes into contact with a disc but receives no pressing force. The motor position corresponding to the contact point is detected using information of the motor current and the motor position. If a piston displacement corresponding to the contact point at which the pad comes into contact with the disc can be accurately detected, the pressing force applied the pad can be more accurately estimated from the displacement of the motor.
The brake devices described in Japanese Unexamined Patent Application Publication Nos. 2003-175816 and 2002-81475 are mounted on vehicle wheels and used in severe environments. Therefore, signals output from sensors used to control the actuators include noise. As a result, if the relationship between the motor current and the motor position is used to construct a graph showing the relationship between the motor position and the pad-pressing force as described in Japanese Unexamined Patent Application Publication No. 2002-81475, the graph differs for each brake device. In addition, the graph varies due to aging of the actuator and ambient temperature variations. Therefore, there is a possibility that the pad-pressing force recognized by the control device will largely differ from the actual pad-pressing force.
In such a case, the left and right wheels receive different braking forces and there is a risk that the driving direction of the vehicle will be changed due to braking, which is extremely dangerous. Therefore, the relationship between the motor position and the pad-pressing force should be adequately compensated, properly corrected to ensure safe braking.